In his book, Unweaving The Rainbow, Richard Dawkins boasts that he tried to tell a six-year-old child that Father Christmas didn't exist. His argument was that Father Christmas would not be able to climb down all those chimneys and tiptoe noiselessly to the bedsides of hundreds of millions of children, all in one night. There simply wouldn't be enough time, even if reindeer were hypersonic.

Well, apart from being a somewhat cruel thing to do to a small child (the distinguished professor for the public understanding of science should pick on someone his own size), the argument is, to be charitable, an incomplete explanation. Dawkins may be no slouch when it comes to evolution, but he knows (by his own admission) rather less about physics. Of course Father Christmas exists, and he can visit arbitrarily as many children has he pleases in as short a time as is convenient, barring mid-air reindeer pile- ups. The reason is that Father Christmas is a Macroscopic Quantum Object.

Let me explain. It is a feature of the quantum world that particles - such as electrons - can be in more than one place at a time, provided that nobody is watching. In a famous experiment known as the "two-slit" test, physicists have been able to fire a single particle at an opaque plate with two separate slits in it. The diffraction pattern seen on the other side of the slits suggests that the particle passes through both holes at once and interacts with itself. However, if detectors are placed at the slits, to see which slit the particle passes through, the diffraction pattern disappears, and the particle can be seen to pass through either one slit or the other, but not both.

The key lies in the fact of observation. Provided that nobody seeks to measure the effect with more than a certain amount of precision, the particle keeps all its options open. But if someone looks too closely, the particle makes its choice. In the language of physics, its quantum wavefunction collapses.

Now, let's think of Father Christmas as a particle, obeying the rules of the quantum world. Following the logic of the two-slit experiment, it is perfectly possible for him to visit all the good children of the world simultaneously, provided that he does so unseen. If he is spotted, his wavefunction will collapse and he will be revealed as your Dad with a comedy beard after all. The quantum nature of Father Christmas explains the taboo against seeing him do his job - which Dawkins does not explain.

But there's more. It is possible to object that Father Christmas is far too large, rubicund and jolly to be a particle. In the real-life, macroscopic world of people, elves and flying reindeer, the quantum behaviour of each of the squillions of particles from which we are made averages out, so what we see is the everyday phenomenon of causes preceding effects, and people who can never be in two places at once.

Cynics might attribute this last consequence to the deficiencies of Railtrack, but it is a fact that real people, even bearded men with red hats and big boots, tend to be found in discrete locations, irrespective of whether they are being watched or not.

This objection doesn't wash, however, because it is possible to have macroscopic quantum objects that are larger than single particles. Scientists have managed to choreograph large clusters of atoms to behave as if they were just one particle, in a kind of nanoscopic Busby Berkeley routine. Admittedly, these clusters are too small to see with the naked eye, let alone qualify as cheerful red- faced men with sacks full of gifts, but the point is made.

Importantly, these macroscopic quantum objects observe the rules of the quantum world when cooled to within a whisker of absolute zero - minus 273 C. Any warmer than this, and the choreography breaks down and the clusters behave like any old bunch of atoms.

Nevertheless, in this frigidity might lie an explanation for another feature of Father Christmas that Dawkins neglects to explain - the undeniable fact that Father Christmas traditionally inhabits cold places, such as Lapland or the North Pole. OK, so neither of these places gets as chilly as absolute zero, but it must count for something that no deserving child would address their wish list to hot places such as, say, Borneo or Brazil. The very idea is quite ridiculous. QED (which stands for Quantum Electrodynamics, as any fule kno.)